1. Field of Invention
The present invention relates to color television systems. More particularly, the present invention relates to a motion detection circuit and method for video decoders.
2. Description of Related Art
In composite video television systems such as NTSC and PAL, luminance and chrominance information share a portion of the total signal bandwidth. In NTSC, for example, chrominance information is encoded on a sub-carrier of 3.579545 MHz. Within the chrominance band which extends from roughly 2.3 MHz to 4.2 MHz, both the chrominance and luminance spectra are intermingled. A television decoder extracts both luminance information and chrominance information from composite signals. Since different comb filters are separately needed for decoding static pictures and motion pictures, a decision circuit, called a motion detection circuit, is used to decide whether the pictures are moving or not.
A composite NTSC video signal can be simply represented as a combination of luminance information (Y) and chrominance information (C). Generally, the luminance information is of the lower frequency components of the composite NTSC video signal, and the chrominance information is of the higher frequency components of the composite NTSC video signal.
FIG. 1 illustrates a schematic view of composite NTSC video signals of three successive frames in a conventional signal encoding manner. The composite NTSC video signals of the first, second and third frames are encoded as Y+C, Y−C and Y+C, respectively, in consideration of the sub-carrier balance. In other words, the luminance information and chrominance information of adjacent frames are of opposite phase relations.
FIG. 2A is a schematic circuit diagram of a conventional motion detection circuit 200, which has two processing paths for respectively detecting luminance and chrominance information of video signals. A video signal comprising both luminance and chrominance information is received and passes through frame delays 202 and 212. The difference in luminance information between two successive frames is determined by a subtracter 203, rectified by a digital filter 204 and an absolute circuit 206 (i.e. for obtaining absolute value), and compared to a reference value in a comparator 208.
FIG. 2B is a partial circuit diagram of the chrominance processing path in FIG. 2A, especially illustrating the processing of the video signals of successive frames. The difference in chrominance information between two frames Y+C3 and Y+C1 spaced one frame apart as Y−C2 is determined by a subtracter 213, rectified by an absolute circuit 216, and compared with another reference value in a comparator 218. A logic gate 210 decides whether the successive video signals are moving or not according to at least one of the first comparing value and the second comparing value from the comparators 208 and 218.
Since the video signals of adjacent frames have opposite phase relations of luminance and chrominance information, the conventional motion detection circuit 200 needs two frame delays 202 and 212 to deal with the chrominance information. However, the frame buffers of the frame delays are very large and expensive, such that the conventional motion detection circuit is inadequate for products.